Blindness

Definitions of blindness and partial sight vary. Blindness is the inability to see. In the UK blindness is defined as a corrected visual acuity of 3/60 or less, or a visual field of no more than 20 degrees in the better eye.

Causes

The loss of vision can result from injury to degeneration of the eyeball or disease; the optic nerve or the nerve pathways that connect the eye to the brain; or the brain itself.

Eyeball

Normal vision depends on an uninterrupted passage of light from the front of the eye to the light sensitive retina at the back. Anything that prevents light from reaching the retina can result in blindness.

A variety of disorders can lead to the clouding of the cornea at the front of the eye. These disorders include Sjogren’s syndrome (in which the eyes become excessively dry), vitamin A deficiency, infections, chemical damage and injury. Corneal ulcers which most commonly develop after severe infections, can lead to blindness due to the scarring of the cornea. Uveitis (inflammation of the iris, choroid, ciliary body) can also cause loss of vision.

Cataract (cloudiness of the lens) is another frequent cause of blindness. It is often the result of the lens becoming less transparent in old age, but can be occasionally present from birth or develop in childhood.

Diabtetes mellitus, hypertension (high blood pressure), or injury can all cause bleeding into the cavity of the eyeball and a subsequent loss of vision. Bleeding into the fluid behind the lens or in front of the lens can also result in loss of vision.

Disorders of the retina that can result in blindness include age related macular degeneration (degeneration of the central area of the retina which occurs in old age); retinopathy due to diabetes or to hypertension; retinal artery occlusion or retinal vein occlusion (blockage of the blood flow to and from the retina); retinal detachment; certain types of tumour, such as malignant melanoma affecting the eye and retinoblastoma; and retina haemorrhage, caused by diabetes, hypertension, injury or vascular disease.

In glaucoma, excessive fluid pressure in the eyeball causes degeneration of nerve fibres at the front end of the optic nerve.

Nerve pathways

The light energy that is received by the retina is transformed into nerve impulses that travel across the optic nerve and nerve pathways to the brain. If the conduction of these nerve impulses is impaired this can result in loss of vision.

Reasons for damage to nerve pathways include pressure caused by a tumour in the orbit (the bony cavity that contains the eyeball); a reduced blood supply to the optic nerve, that may be caused by diabetes mellitus, a tumour injury, hypertension, or temporal arteritis (inflammation of arteries in the scalp); optic neuritis (inflammation of the optic nerve that may occur in multiple sclerosis); the toxic effects of certain chemicals; and certain nutritional deficiencies.

Brain

The nerve impulses from the retina eventually arrive in a region of the cerebrum (the main mass of the brain) caused the visual cortex. If there is pressure on the visual cortex from a brain tumour or brain haemorrhage, or if the blood supply to the visual cortex has been reduced following a stroke, this can lead to blindness.

Diagnosis and treatment

It is often possible to detect the cause of blindness by direct examination of the eye, using such techniques as slit-lamp examination, tonometry and ophthalmoscopy. The conduction of nerve impulses can be measured by means of evoked responses.

The treatment of blindness will depend on the underlying cause. If the loss of vision cannot be corrected, the patient may then be registered as legally blind or partially sighted, and will become eligible for certain services and benefits.